Preparation of a dimethylnaphthalene

ABSTRACT

A method for preparing one or more diemthylnaphthalenes from one or more dimethyltetralins, and optionally for preparing one or more other specific dimethylnaphthalenes by isomerization of the aforesaid dimethylnaphthalene(s) is disclosed.

RELATED APPLICATION

This application is a continuation-in-part of pending patent applicationSer. No. 316,309, filed Feb. 27, 1989, now U.S. Pat. No. 5,012,024,which is a continuation-in-part of patent application Ser. No. 210,999,filed Jun. 24, 1988, and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for preparing one or morespecific dimethylnaphthalene isomers by dehydrogenating one or morespecific dimethyltetralin isomers.

2. Description of the Prior Art

Naphthalene dicarboxylic acids are monomers that are known to be usefulfor the preparation of a variety of polymers. For example, poly(ethylene2,6-naphthalate) prepared from 2,6-naphthalene dicarboxylic acid andethylene glycol has better heat resistance and mechanical propertiesthan polyethylene terephthalate and is useful in the manufacture offilms and fibers.

Dimethylnaphthalenes are desirable feedstocks for oxidation to thecorresponding naphthalene dicarboxylic acids. A known conventionalprocess for producing a naphthalene dicarboxylic acid comprises theoxidation of a dimethylnaphthalene with oxygen in the liquid phase in anacetic acid solvent at an elevated temperature and pressure and in thepresence of a catalyst comprising cobalt, manganese and brominecomponents.

Typically dimethylnaphthalenes are found in refinery or coal-derivedstreams as mixtures of all of the ten possible dimethylnaphthaleneisomers. However, separation of these isomers is very difficult andexpensive. Consequently, methods for producing specificdimethylnaphthalenes or mixtures of two or three specificdimethylnaphthalenes in high purity and quality are highly desirable.One type of such method is a multistep synthesis involving (1) theformation of an alkenylbenzene by the reaction of o-, m- or p-xylene orethylbenzene with butadiene, (2) the cyclization of the resultingalkenylbenzene to form one or more dimethyltetralins belonging to one ortwo of three groups of three isomeric dimethyltetralins --that is,either group A containing the 1,5-, 1,6-, 2,5- and2,6-dimethyltetralins, group B containing the 1,7-, 1,8-, 2,7- and2,8-dimethyltetralins, or group C containing the 1,3-, 1,4-, 2,3-, 5,7-,5,8- and 6,7-dimethyltetralins--(3) the dehydrogenation of thedimethyltetralin(s) to form the corresponding dimethylnaphthalene(s),and (4) the isomerization of the resulting dimethylnaphthalene(s) to thedesired specific dimethylnaphthalene.

For example, Thompson, U.S. Pat. Nos. 3,775,496; 3,775,497; 3,775,498;3,775,500 disclose processes for the cyclization of specificalkenylbenzenes to one or more specific dimethyltetralins at 200°-450°C. in the presence of any suitable solid acidic cyclization catalystsuch as acidic crystalline zeolites as well as silica-alumina,silica-magnesia and silica-alumina-zirconia and phosphoric acid,followed by the dehydrogenation of the resulting dimethyltetralin(s) inthe vapor state to the corresponding dimethylnaphthalene(s) in ahydrogen atmosphere at 300°-500° C. and in the presence of a soliddehydrogenation catalyst such as noble metals on carriers andchromia-alumina, and thereafter isomerization of each of the aforesaiddimethylnaphthalene(s) to the desired isomer within the triad ofdimethylnaphthalenes to which the isomer being isomerized belongs, at275°-500° C. in the presence of a solid acidic isomerization catalyst ofthe same type as described in respect of the cyclization disclosedtherein. In the alternative, both the cyclization and isomerizationreactions can be performed in the liquid phase, in which case thecyclization is performed at 200°-275° C. with a solid phosphoric acidcatalyst, at 70°-140° C. with an acidic ion exchange resin, an acidiccrystalline zeolite, hydrofluoric or sulfuric acid as the catalyst or asiliceous cracking catalyst.

More specifically, Thompson, U.S. Pat. No. 3,775,496 discloses thecyclization of 5-(m-tolyl) -pent-2-ene to 1,6- and1,8-dimethyltetralins, which are then dehydrogenated to 1,6- and1,8-dimethylnaphthalenes, which in turn are isomerized to 2,6- and2,7-dimethylnaphthalenes, respectively. Thompson, U.S. Pat. No.3,775,497 discloses the cyclization of 5-phenyl-hex-2-ene to1,4-dimethyltetralin which is then dehydrogenated to1,4-dimethylnaphthalene, which is in turn isomerized to2,3-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,498 discloses thecyclization of 5-(o-tolyl)-pent-2-ene to 1,5-dimethyltetralin, which isthen dehydrogenated to 1,5-dimethylnaphthalene, which is in turnisomerized to 2,6-dimethylnaphthalene. Thompson, U.S. Pat. No. 3,775,500discloses the cyclization of 5-(p-tolyl)-pent-2-ene to1,7-dimethyltetralin, which is then dehydrogenated to1,7-dimethylnaphthalene, which in turn is isomerized to2,7-dimethylnaphthalene.

Shimada et al., U.S. Pat. No. 3,780,119 disclose a method for theisomerization of dimethylnaphthalene by the use at a temperature above260° C. of a mordenite catalyst in which a metal form of mordenite is inexcess of 20 weight percent of the mordenite, with the metal beingselected from the group consisting of lithium, sodium, potassium,magnesium, calcium, strontium, barium, zinc and aluminum.

Suld et al., U.S. Pat. No. 3,803,253 disclose a method for thehydroisomerization of a dimethylnaphthalene by the use of a combinationof a hydrogenation catalyst and a calcium-containing zeolite catalyst,such as a calcium-exchanged synthetic faujasite, for example, a Y-typemolecular sieve.

Shima et al., U.S. Pat. No. 3,806,552 disclose a method for theisomerization of dimethylnaphthalenes in the gas phase by the use of amixed catalyst consisting of (a) 65-95 weight percent of a hydrogen formof mordenite in which above 80 weight percent of the metal cations arereplaced with hydrogen ions, and (b) 5-35 weight percent of catalystselected from the group consisting of bentonite and fuller's earth.

Hedge, U.S. Pat. No. 3,855,328 discloses a method for the isomerizationof dimethylnaphthalenes by the use of a Type Y alumino silicate zeoliteat 120°-300° C. in the liquid phase. The catalysts havealuminum-to-silicon atomic ratios of 0.1-1.0.

Ogasawara et al., U.S. Pat. No. 3,888,938 disclose a method for theisomerization of dimethylnaphthalenes in the liquid phase by the use ofa mixed catalyst consisting of (a) 70-95 weight percent of a hydrogenform of mordenite in which above 80 weight percent of the metal cationsare replaced with hydrogen ions, and (b) 5-30 weight percent of apromoter selected from the group consisting of bentonite and fuller'searth.

Hedge et al., U.S. Pat. No. 3,928,482 disclose the isomerization ofeither dimethyldecalins, dimethyltetralins or dimethylnaphthalenes inthe presence of an alumino silicate zeolite containing polyvalent metalcations in exchange positions, such as a rare earth-exchanged Type Yzeolite.

Yokayama et al., U.S. Pat. No. 3,957,896 disclose the selectiveisomerization of dimethylnaphthalenes in the presence of any kind ofnatural or synthetic, solid acid catalyst, such as Y-type zeolite aswell as silica-alumina, silica-magnesia, silica-zirconia,silica-aluminazirconia, fuller's earth, natural or synthetic mordenite,X-type zeolite, A-type zeolite and L-type zeolite. These catalysts maybe substituted partly or wholly by hydrogen or metal. Furthermore, thesecatalysts can be unsupported or supported on carriers.

Onodera et al., U.S. Pat. No. 4,524,055 discloses a crystallinealuminosilicate zeolite that is useful in the isomerization ofdimethylnaphthalenes and has a silica-to-alumina mole ratio of 10 to100, specific x-ray lattice distances, and a specificcylohexane-to-n-hexane adsorption ratio of at least 0.7.

Maki et al., U.S. Pat. No. 4,556,751 disclose the isomerization ofdimethylnaphthalenes in the presence of a crystalline aluminosilicatehaving a pentasil structure and a silica-to-alumina molar structure of12 or higher. In addition, the aluminosilicate may contain some othermetals as non-exchangeable metals.

A problem in all such prior art methods is the presence of otherdimethylnaphthalene isomers and unconverted dimethyltetralin asimpurities and by-products in the finally obtained, desired specificdimethylnaphthalene isomer. The presence of such impurities andby-products markedly reduces the utility and commercial value of thedesired dimethylnaphthalene isomer, especially as a precursor for theformation of a naphthalene dicarboxylic acid for use as a monomer in themanufacture of a polymer. In addition, catalysts tend to deactivaterelatively rapidly at the high temperatures employed in vapor phaseprocesses. Therefore, it is highly desirable to employ liquid phaseprocesses and to improve the completeness of each step in the aforesaidmultistep synthesis and the selectivity of each step therein for theproduction of the desired product therefrom.

In this regard, it is known that in the presence of an acid catalyst,the dimethylnaphthalene isomers are isomerizable within each triad ofdimethylnaphthalene isomers--that is, within the 1,5-, 1,6- and2,6-dimethylnaphthalenes of triad A, within the 1,7-, 1,8-, and2,7-dimethylnaphthalenes of triad B, and within the 1,3-, 1,4- and2,3-dimethylnaphthalenes of triad C. It is also known that theinterconversion of a dimethylnaphthalene isomer within one of theaforesaid triads to a dimethylnaphthalene isomer within another of theaforesaid triads occurs to a relatively lesser extent. However, it ishighly desired to improve the selectivity and completeness of theaforesaid dehydrogenation and isomerization steps in the aforesaidmultistep synthesis for the formation of the specificdimethylnaphthalene isomer(s) desired.

OBJECTS OF THE INVENTION

It is therefore a general object of the present invention to provide animproved method for manufacturing with an improved yield and selectivitya specific dimethylnaphthalene isomer or set of dimethylnaphthaleneisomers which meets the aforementioned requirements for selectivity andcompleteness and catalyst activity.

It is a related object of the present invention to provide an improvedmethod for manufacturing with an improved yield and selectivity aspecific dimethylnaphthalene isomer or set of dimethylnaphthaleneisomers by the dehydrogenation of a specific dimethyltetralin isomer ora set of dimethyltetralin isomers, and optionally then isomerizing theresulting first dimethylnaphthalene(s) to one or more otherdimethylnaphthalene isomers that belong to the same triad or triads asdo the first dimethylnaphthalene isomer(s).

Other objects and advantages of the method of the present invention willbecome apparent upon reading the following detailed description andappended claims.

SUMMARY OF THE INVENTION

The objects are achieved by an improved method for preparing adimethylnaphthalene comprising: contacting a first feedstock comprisingat least one dimethyltetralin in liquid form with a soliddehydrogenation catalyst in a reaction vessel at an elevated temperatureand at a pressure that is sufficiently high to maintain the firstfeedstock substantially in the liquid phase, to thereby effectconversion of the aforesaid dimethyltetralin in an equilibriumdehydrogenation reaction to form hydrogen and a first liquid productcomprising a dimethylnaphthalene formed from each aforesaiddimethyltetralin, and removing a substantial portion of the hydrogenbeing formed in the dehydrogenation reaction from the reaction vessel tothereby shift the aforesaid equilibrium toward the formation of thedimethylnaphthalene product.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Any dimethyltetralin or mixture of dimethyltetralins is suitable for useas a feedstock in the method of this invention. In the method of thepresent invention, the dehydrogenation step is followed preferably by anisomerization step in which the dimethylnaphthalene(s) produced in thedehydrogenation step is isomerized to the desireddimethylnaphthalene(s). Thus, if a particular dimethylnaphthalene or setof dimethylnaphthalenes is desired as the final product, then it ispreferred to use a dimethyltetralin or set of dimethyltetralins thatform by the dehydrogenation of the method of the present inventioneither (1) such dimethylnaphthalene(s) directly or (2) otherdimethylnaphthalene(s) which belong to the same triad or triads ofdimethylnaphthalene isomers to which the desired dimethylnaphthaleneisomer(s) belong, and which can then be isomerized in the preferredmethod of this invention to the desired dimethylnaphthalene(s).

In addition or in the alternative, the specific dimethyltetralin or setof specific dimethyltetralins employed in the dehydrogenation step ofthe method of this invention can depend on the manner in which eachdimethyltetralin employed is obtained. For example copending U.S. patentapplication Ser. No. 316,308, filed Feb. 27, 1989 and Sikkenga, Lamb,Zaenger and Williams, copending U.S. patent application Ser. No.556,297, filed concurrently herewith (the entire disclosures of whichare specifically incorporated herein by reference), disclose theformation of one or more dimethyltetralins by the cyclization of 5-(o-,m-, or p-tolyl)-pent-1- or -2-ene or 5-phenyl-hex-1- or -2-ene, andthereafter the dehydrogenation of the resulting dimethyltetralin(s) toform one or more corresponding dimethylnaphthalenes.

When 5-(o-tolyl)-pent-1- or -2-ene is the feedstock to the aforesaidcyclization step, 1,5-, 1,6-, 2,5-, or 2,6-dimethyltetralin or a mixturethereof comprises at least 80, preferably at least 85 weight percent ofthe dimethyltetralins produced therefrom, which resultingdimethytetralins are in turn the feedstock and are converted in thedehydrogenation step of the present invention to the corresponding 1,5-,1,6- and 2,6-dimethylnaphthalenes, which are then the feedstock in theisomerization step of the preferred embodiment of the present inventionand are converted therein to 2,6-dimethylnaphthalene.

When 5-(m-tolyl)-pent-1- or -2-ene is the feedstock to the aforesaidcyclization step, 1,5- 1,6- 1,7-, 1,8- 2,5- 2,6-, 2,7- or2,8-dimethyltetralin or a mixture thereof comprises at least 80,preferably at least 85 weight percent of the dimethyltetralins producedtherefrom, which dimethyltetralins are in turn the feedstock and areconverted in the dehydrogenation step of the present invention to thecorresponding 1,5-, 1,6-, 1,7-, 1,8- 2,6- and 2,7-dimethylnaphthalenes,which are then the feedstock in the isomerization step of the presentinvention and are converted therein to 2,6- and2,7-dimethylnaphthalenes.

When 5-(p-tolyl)-pent-1- or -2-ene is the feedstock to the aforesaidcyclization step, 1,7-, 1,8-, 2,7- or 2,8-dimethyltetralin or a mixturethereof comprises at least 80, preferably at least 85 weight percent ofthe dimethyltetralins produced therefrom, which dimethyltetralins are inturn the feedstock and are converted in the dehydrogenation step of thepresent invention to the corresponding 1,7-, 1,8- and2,7-dimethylnaphthalenes which are then the feedstock and are convertedin the isomerization step of the present invention to2,7-dimethylnaphthalene.

When 5-phenyl-1- or -2-hexene is the feedstock to the aforesaidcyclization step, 1,3-, 1,4-, 2,3-, 5,7, 5,8-, or 6,7-dimethyltetralinor a mixture thereof comprises at least 80, preferably at least 85weight percent of the dimethyltetralins produced therefrom, whichdimethyltetralins are in turn the feedstock and are converted in thedehydrogentation step of the present invention to the corresponding,1,3-, 1,4- and 2,3-dimethylnaphthalenes, which are then the feedstock inthe isomerization step of the present invention and are converted to2,3-dimethylnaphthalene therein.

In the method of the present invention, each of the aforesaiddehydrogenation reaction and optional isomerization reaction isperformed in the liquid phase at an elevated temperature and at asufficiently high pressure to ensure that the feedstock for theparticular step is maintained substantially in the liquid phase. Byelevated temperature it is meant a temperature sufficiently high so thata significant portion of the feedstock for the respective reaction isconverted to the desired product using preselected catalyst levels andreaction times for batch processes, or preselected space velocities forcontinuous processes. The dehydrogenation reaction is preferablyperformed at a temperature in the range of about 200° C., morepreferably about 220° C., to about 500° C., more preferably to about450° C. Most preferably, the dehydrogenation reaction is performed at atemperature in the range of about 220° C. to about 420° C. Preferably,the dehydrogenation reaction is performed at a pressure in the range ofabout 0.1, more preferably about 1.0, to about 30.0, more preferably toabout 20.0 atmospheres absolute. The isomerization reaction ispreferably performed at a temperature in the range of about 200° C.,more preferably about 240° C., to about 420° C., more preferably toabout 380° C. Most preferably the isomerization reaction is performed ata temperature in the range of about 240° C. to about 350° C. Theisomerization reaction is preferably performed at a pressure in therange of about 0.1, more preferably about 0.5, to about 20.0, morepreferably 5.0 atmospheres absolute.

Each of the dehydrogenation and isomerization reactions can be performedwith or without a solvent for the respective feedstock. Preferably asolvent is not employed in the aforesaid steps. If employed, a solventin any of the aforesaid steps must be inert under the conditionsemployed and suitably comprises a paraffin such as a tetradecane, or anaromatic hydrocarbon such as anthracene, or mixtures thereof, whichpreferably boils above about 270° C.

Each of the dehydrogenation and isomerization steps of the method of thepresent invention can be performed either batchwise or continuously. Thereaction apparatus to be used in each aforesaid step can be of any knowntype such as a fixed bed, moving bed, fluidized bed, liquid phasesuspended bed or a solid-liquid mixture in a stirred tank. Generally,however, the use of a fixed bed is commercially preferred for continuousoperation. When conducting the dehydrogenation reaction of thisinvention in a continuous manner, it is advantageous to use two or morefixed bed reactors in series. Hydrogen formed during the dehydrogenationreaction is preferably removed from the product mixture between suchfixed bed reactors arranged in series.

The improved conversion of the feedstock and selectivity for theproduction of the desired product or set of products for each of thedehydrogenation and isomerization steps of the method of this inventionare the result of the temperature and pressure conditions employed andthe high activity and selectivity of the catalysts employed in eachaforesaid step, which in turn permits the use of less severeconditions--that is, lower temperatures and pressures--such that greaterselectivity and reduced catalyst deactivation can be achieved.

The catalyst employed in the dehydrogenation step of the method of thisinvention is any solid dehydrogenation catalyst that is capable ofeffecting the dehydrogenation and exhibiting a reasonable lifetime underthe conditions employed, including catalysts such as noble metals oncarriers such as reforming catalysts. Aluminas, silicas,alumina-silicas, and activated carbons are examples of suitable carriersor supports. The noble metals include, for example, platinum, palladium,ruthenium and rhenium. The noble metal component can also comprisemixtures of two or more noble metals. Preferably, palladium on an activecarbon or alumina support containing from about 0.5, more preferablyfrom about 1.0, to about 15, more preferably to about 10 weight percentof palladium, calculated as elemental palladium and based on the weightof the catalyst, is employed as the dehydrogenation catalyst.

Other preferred dehydrogenation catalysts include platinum on activatedcarbon or alumina supports, rhenium on activated carbon or aluminasupports and mixtures of platinum and rhenium on activated carbon oralumina supports, wherein the platinum and rhenium are each present fromabout 0.01, preferably 0.05, to about 10.0, preferably 5.0 weightpercent calculated as the element and based on the weight of thecatalyst. A more preferred dehydrogenation catalyst comprises a mixtureof platinum and rhenium on gamma alumina where the platinum and rheniumare each present in the range of about 0.1 to about 10.0 weight percentcalculated as the element, and based on the weight of the catalyst. Asupport material such as an alumina or other non-combustible supportmaterial has an advantage over a carbon support material in that thenon-combustible support can be exposed to air or other source ofoxygen-containing gas at an elevated temperature to regenerate adeactivated catalyst. Consequently, such a catalyst can be cycledwherein between each cycle of use as a dehydrogenation catalyst thecatalyst is regenerated with an oxygen-containing gas at an elevatedtemperature. Preferably the level of oxygen in the oxygen-containing gasis about 1 wt % to about 25 wt %, the regeneration temperature is in therange of about 400° C. to about 600° C. and the time of exposure to theoxygen-containing gas at these temperatures is that sufficient toregenerate the catalyst.

In the liquid phase dehydrogenation reactions of this invention, whenconducted in either a batch or continuous manner, and particularly whenusing the preferred dehydrogenation catalysts, the addition of hydrogento the reaction mixture is not necessary to maintain catalyst activityduring extended catalyst use, i.e., the liquid phase dehydrogenationreaction in the method of this invention wherein a dimethyltetralin isdehydrogenated to a dimethylnaphthalene proceeds in the absence ofhydrogen added to the reaction mixture. Without intending to be bound bya theory of operation, it appears that during the liquid phasedehydrogenation method of this invention wherein dimethyltetralins aredehydrogenated to dimethylnaphthalenes using a dehydrogenation catalyst,and particularly the preferred noble metal dehydrogenation catalystsdisclosed herein, the hydrogen generated during the dehydrogenationreaction effectively maintains catalyst activity.

In the dehydrogenation method of this invention it is advantageous toremove hydrogen during the liquid phase dehydrogenation reaction. Thisis accomplished in a batch procedure by venting the hydrogen from thevessel used to conduct the batch reaction. If operating in a continuousmode, a plurality of series arranged fixed bed reactors can be utilizedwith the hydrogen vented from the process stream between fixed bedreactors. Other suitable methods for venting the hydrogen from a fixedbed reactor can be used.

If the dehydrogenation is performed on a batch basis, the catalyst isemployed at a level in the range of from about 0.005, preferably fromabout 0.01, to about 1.0, preferably to about 0.2 weight percent of thenoble metal component, calculated as the elemental noble metal and basedon the weight of the dimethyltetralin feedstock, and the reaction timeis from about 1, preferably from about 2, to about 50, preferably toabout 40 hours. If the dehydrogenation is performed on a continuousbasis, the space velocity is in the range of from about 0.1, preferablyfrom about 10, to about 5000, preferably to about 2000 parts of thedimethyltetralin feedstock per part of the noble metal component(calculated as the elemental noble metal) of the catalyst by weight perhour.

The catalyst employed in the isomerization step of the method of thisinvention comprises either beta zeolite or an acidic ultrastable--thatis, a thermally stabilized or dealuminated--Y-type crystallinealuminosilicate zeolite having a silica-to-alumina molar ratio of fromabout 4:1 preferably from about 5:1, to about 10:1, preferably to about6:1, and having pore windows provided by twelve-membered ringscontaining oxygen, and a unit cell size of from about 24.2, preferablyfrom about 24.3, to about 24.7, preferably to about 24.6 angstroms. Asuitable such zeolite is marketed by Union Carbide under the name LZ-Y72or LZ-Y20. Water is not detrimental to catalytic activity or selectivityin the isomerization process.

The isomerization catalyst preferably comprises beta zeolite. Thecomposition, structure and preparation of beta zeolite are described inWadlinger et al., U.S. Pat. No. 3,308,069 which in its entirety isspecifically incorporated herein by reference. The structure of betazeolite is also reported in J. Haggin, "Structure of Zeolite BetaDetermined," in Chemical & Engineering News, p. 23 (Jun. 20, 1988). Betazeolite is also commercially available from PQ Corporation.

The aforesaid ultrastable Y-type zeolite which can be employed in thecatalyst for the isomerization step of the method of this invention isin the hydrogen form and contains from about 0.01, preferably from about1, up to about 5, preferably up to about 3, weight percent of sodium,calculated as elemental sodium and based on the weight of the zeolite.

Preferably the isomerization catalyst comprises a hydrogenationcomponent comprising a Group VIII metal, which more preferably ispalladium, platinum or nickel.

The aforesaid zeolite of the isomerization catalyst can be employedeither unsupported or supported on a porous refractory, inorganic oxidethat is inert under the conditions employed, such as silica, alumina,silica-alumina, magnesia, bentonite or other such clays. If a support isemployed, preferably the support comprises silica, alumina orsilica-alumina. When a support is employed, the zeolite comprises fromabout 10, preferably from about 20, to about 90, preferably to about 80weight percent based on the weight of the catalyst.

If the isomerization is performed on a batch basis, the catalyst isemployed at a level in the range of from about 0.1, preferably fromabout 1.0, to about 5, preferably to about 3 weight percent of thezeolite component of the catalyst, based on the weight of thedimethylnaphthalene feedstock, and the reaction time is from about 0.5,preferably from about 2, to about 10, preferably to about 6 hours. Ifthe isomerization is performed on a continuous basis, the space velocityis in the range of from about 0.1, preferably from about 0.5 to about20, preferably to about 10 parts of dimethylnaphthalene feedstock perpart of zeolite component of the catalyst by weight per hour.

For the dehydrogenation and isomerization reactions describedhereinabove, it is preferable to conduct each reaction at the lowestpossible reaction temperature that provides for the conversion of asubstantial portion of the reaction feedstock to the respective product.At elevated reaction temperatures, coke, tars and other reactionsideproducts tend to form more rapidly and deposit on and deactivate thecatalysts disclosed herein. However, regardless of the reactiontemperature used, as the catalyst ages catalytic activity typicallydecreases. This decrease in catalyst activity, which results in reducedfeedstock conversion at preselected reaction conditions such as reactionpressure, catalyst level, space velocity and reaction temperature, canbe offset somewhat by increasing the reaction temperature. Consequently,a preferred procedure for maximizing the useful life of thedehydrogenation and isomerization catalysts of this invention is tobegin using the catalysts at as low a reaction temperature that providesfor the conversion of a significant portion of the respective feedstockand then increase the temperature of the reaction as the catalyst agesso as to maintain desirable feedstock conversion levels. For example,when using a batch process, the temperature of the reaction can beraised with each successive batch. When using a continuous process, thereaction temperature of the catalyst bed or continuous stirred tankreactor can be raised as the catalyst ages. When employing an ageddehydrogenation catalyst in the method of this invention, a reactiontemperature greater than 300° C. is suitable for maintaining theconversion of a significant portion and preferably at least about 50 wt% and more preferably at least about 70 wt % of the dehydrogenationreaction feedstock to the desired product or products. Preferably thistemperature is in the range of from about 305° C. to about 500° C., andmore preferably in the range of from about 310° C. to about 450° C. Whenusing an aged isomerization catalyst in the method of this invention, areaction temperature greater than 300° C. is suitable for maintainingthe conversion of a significant portion and preferably at least about 20wt % of the isomerization reaction feedstock to the desired product orproducts. Preferably this temperature is in the range of from about 305°C. to about 420° C., more preferably in the range of from about 310° C.to about 380° C.

U.S. patent application Ser. No. 316,309 filed on Feb. 27, 1989 ishereby specifically incorporated by reference.

The present invention will be more clearly understood from the followingspecific examples.

EXAMPLES 1-6

In each of Examples 1-6, the liquid feed and a 5 weight percentpalladium-on-carbon catalyst were charged to a flask and nitrogen wascontinuously passed through the reaction mixture to remove oxygen. Thetemperature of the reaction mixture was raised to the reactiontemperature, and periodically samples were removed from the flask andanalyzed. Hydrogen generated by the reaction was permitted to vent fromthe flask during the reaction. The experimental conditions employed, thecompositions of the feedstock employed and of the resulting productscontaining up to 13 carbon atoms, the percent conversion of thefeedstock, and the percent selectivity of the formation of desiredproduct from the total amount of feedstock converted in each of Examples1-6 are presented in Table 1.

The results in Table 1 illustrate that even with the mild temperatureand pressure conditions employed in Examples 1-6, the dehydrogenation ofthe method of this invention affords both excellent conversion andselectivity.

                                      TABLE 1                                     __________________________________________________________________________               Feed                                                                              Example 1                                                                             Example 2                                                                             Feed                                                                              Ex. 3                                                                             Feed                                                                              Ex. 4                                                                             Feed                                                                              Ex. 5                                                                             Feed                                                                              Ex.                __________________________________________________________________________                                                               6                  Conditions                                                                    Hours on stream                                                                              2.0 4.8 6.0 8.3     8.5     3.0     3.0     6.3                Catalyst cycle no.                                                                           1   1   1   1       1       1       1       1                  Temperature (°C.)                                                                     242 243 245 244     251     253     254     254                Pressure (psig)                                                                              1.0 1.0 1.0 1.0     0.0     0.0     0.0     1.0                Feed/catalyst  10.0                                                                              10.0                                                                              50.0                                                                              50.0    100.0   49.9    100.0   10.0               weight ratio                                                                  Compositions (wt %)                                                           1,4-DMT    0.0 0.0 0.0 0.0 0.0                 92.7                                                                              0.4                        1,5-DMT    92.0                                                                              2.2 0.0 4.2 1.9 93.9                                                                              1.1 0.5             1.2 0.0                1,6-DMT    0.0 0.0 0.0 0.0 0.0 0.6     0.8             55.2                                                                              0.0                1,7-DMT    0.0 0.0 0.0 0.0 0.0         89.4                                                                              0.4                                1,8-DMT    0.0 0.0 0.0 0.0 0.0                         36.0                                                                              1.4                other DMTs 0.1 0.4 0.2 0.3 0.0 1.2 0.3 0.1 0.0 0.0 3.6 1.0 0.0                m-xylene   0.1 0.6 0.5 0.5 0.3 0.1 0.3 0.0 0.0             0.4                non-cyclic 4.9 4.5 3.9 4.3 4.3 2.8 1.7 7.5 6.7 5.3 5.0 3.7 2.2                Products                                                                      1,3-DMN    0.0 0.0 0.0 0.0 0.0     0.0 0.0 0.0 0.2 0.6     0.0                1,4-DMN    0.0 0.0 0.0 0.0 0.0     0.0 0.0 0.2 1.6 90.2    0.0                1,5-DMN    0.9 89.8                                                                              93.2                                                                              88.4                                                                              92.5                                                                              0.7 94.3                                                                              0.0 0.5             1.3                1,6-DMN    0.0 0.5 0.5 0.4 0.0     0.8 0.0 0.8         0.9 58.2               1,7-DMN    0.0 0.0 0.0 0.0 0.0     0.1 1.5 90.5            0.6                1,8-DMN    0.0 0.0 0.6 0.2 0.0     0.4 0.0 0.1             35.6               2,6 + 2,7-DMNs                                                                           0.0 0.0 0.0 0.0 0.0     0.0 0.0 0.3             0.2                Lights     0.1 1.5 1.0 1.1 0.6     0.7 0.0 0.1 0.4         0.3                Heavies    0.1 0.0 0.0 0.0 0.0     0.0 0.0 0.0         1.8 0.0                Other      1.8 0.5 0.1 0.5 0.4 0.6 0.4  0.0                                                                              0.2     0.3     0.0                Total      100.1                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             99.9                                                                              99.9                                                                              100.2                                                                             100.0                                                                             99.7                                                                              100.1              Total DMNs     90.3                                                                              94.3                                                                              89.0                                                                              92.5    95.5                                                                              1.5 92.4    90.8    95.8               % Conversion   97.6                                                                              100.0                                                                             95.4                                                                              98.0    98.8    99.5    99.5    98.5               % Selectivity  99  100 100 101     101     100     96.0    103.4              __________________________________________________________________________

EXAMPLES 7-24

In each of Examples 7-24, the particular isomer of dimethylnaphthaleneemployed as the feed was mixed in liquid form with unsupported catalystin a stirred reaction vessel with a continuous nitrogen purge topreclude oxygen from the system. The temperature of the reaction vesselwas raised to the reaction temperature and samples were withdrawn atvarious times after commencement of the reaction and analyzed. Theconditions employed, the compositions of the feedstock employed and ofthe resulting products containing up to 13 carbon atoms, the percentconversion of the feedstock, and the percent selectivity of theformation of desired product from the total amount of feedstockconverted in each of Examples 7-24 are presented in Table 2.

The catalyst employed in Example 7 was a crystalline borosilicatemolecular sieve (HAMS-1B from Amoco Chemical). The catalyst employed inExample 8 was Union Carbide's LZ-Y20 ultrastable Y-type sieve,containing 2 weight percent of copper, calculated as elemental copper.The catalyst employed in Example 9 was Union Carbide's LZ-Y62, anon-ultrastable, Y-type sieve in the ammonia exchanged form and having aunit cell size of 24.73 Å. The catalyst employed in Examples 10 and 11was commercially available Union Carbide's LZ-Y82, anultra-stable-molecular sieve having a unit cell size of 24.56 Å and asodium content of less than 0.2 weight percent. In Example 10, the sievewas in the ammonia form and had not been calcined. In Example 11, thesieve had been calcined to form the hydrogen form. The catalyst employedin Example 12 was a commercially available amorphous silica-aluminacontaining 13 weight percent of alumina. The catalyst employed inExample 13 was commercially available mordenite in the acid form. Thecatalyst employed in Examples 14, 15 and 17-22 was commerciallyavailable Union Carbide's LZ-Y72 in the hydrogen form as received fromthe manufacturer. The catalyst employed in Example 16 was commerciallyavailable Grace USY sieve containing 2.6 weight percent of sodium andhas chemical and physical properties that are very similar to those ofUnion Carbide's LZ-Y72, and is also suitable for use as a catalyst ineither the cyclization or isomerization step in the method of thisinvention. 1,5-dimethylnaphthalene was the feed in Examples 7-21. Thefeed was 1,7-dimethylnaphthalene in Example 22 and1,4-dimethylnaphthalene in Examples 23 and 24. For the purposes of Table2, the concentration of 2,7-DMN in the product is taken to beapproximately equal to the concentration of 1,7-DMN and is subtractedfrom the sum of 2,6-DMN and 2,7-DMN (which are determined together) forthe purpose of determining the concentration of 2,6-DMN alone. Theeffective maximum concentrations of a particular desired DMN in itstriad is its equilibrium concentration in the triad, which generally is40-45 weight percent.

                                      TABLE 2                                     __________________________________________________________________________                    Feed                                                                              Ex. 7                                                                             Feed                                                                              Ex. 8                                                                             Feed                                                                              Ex. 9                                                                              Ex. 10                                                                              Ex. 11                                                                             Ex.                                                                                Ex.                  __________________________________________________________________________                                                             13                   Conditions                                                                    Temperature (°C.)                                                                          249     243     248  249   240  233  248                  Pressure (psig)     1       1       1    1     1    1    1                    Catalyst            Amsac-  LZ-20   LZ-Y62                                                                             LZ-Y82.sup.1                                                                        LZ-Y82.sup.2                                                                       SiO.sub.2 /                                                                        Morde-                                   3400    2% Cu                   Al.sub.2 O.sub.3                                                                   nite.sup.4           Feed/catalyst wt ratio                                                                            10      10      10.0 9.9   9.8  10.1 9.8                  Hours on stream     7.3     13      12   11.8  5.5  13   11.5                 Product Composition (wt %)                                                    1,2-DMN         0.0 0.0 0.0 0.1 0.0 0.0  0.0   0.4  0.0  0.0                  1,3-DMN         0.0 0.0 0.0 0.0 0.0 0.0  0.0   0.0  0.0  0.0                  1,4-DMN         0.0 0.0 0.0 0.0 0.0 0.0  0.0   0.0  0.0  0.0                  1,5-DMN         93.4                                                                              69.3                                                                              82.6                                                                              8.7 82.6                                                                              82.3 75.6  4.1  21.9 41.2                 1,6-DMN         0.0 20.1                                                                              11.8                                                                              37.8                                                                              11.8                                                                              12.1 18.3  25.4 42.8 29.4                 1,7-DMN         0.0 0.0 1.2 1.5 1.2 1.3  1.2   3.6  1.2  0.7                  2,3-DMN         0.0 0.0 0.0 0.0 0.0 0.0  0.0   1.2  0.0  0.0                  2,6- + 2,7-DMNs 0.0 5.4 1.8 36.3                                                                              1.8 2.1  3.2   30.4 29.3 27.6                 Lights          6.3 2.6 1.7 1.4 1.7 1.8  1.2   1.3  1.2  1.0                  Heavies         0.0 1.9 0.2 5.8 0.2 0.1  0.1   16.2 1.2  0.0                  Naphthalene     0.0 0.0 0.0 0.1 0.0 0.0  0.0   0.7  0.0  0.0                  Methylnaphthalenes                                                                            0.0 0.2 0.6 6.5 0.6 0.3  0.2   12.7 2.0  0.1                  Other           0.2 0.5 0.0 1.9 0.0 0.0  0.0   3.9  0.5  0.0                  Total           99.9                                                                              100.0                                                                             99.9                                                                              100.1                                                                             99.6                                                                              100.1                                                                              99.8  100.1                                                                              100.1                                                                              100.0                Total DMNs      93.4                                                                              94.8                                                                              97.3                                                                              84.4                                                                              97.3                                                                              97.8 98.3  65.1 95.2 98.9                 2,7-DMN %                                                                     2,6-DMN % in the 1,5-, 1,6- and                                                               0.0 5.7 0.6 42.8                                                                              0.6 0.9  2.1   47.7 30.3 27.6                 2,6-DMN triad                                                                 2,6-DMN selectivity 100     71.7               40.5 93.1 >100                 __________________________________________________________________________     Footnotes                                                                     .sup.1 not calcined                                                           .sup.2 calcined                                                               .sup.3 13% Al.sub.2 O.sub.3                                                   .sup.4 in H form                                                         

                    Ex. 14                                                                              Ex. 15                                                                              Ex. 16                                                                             Ex. 17                                                                               Ex. 18                                                                             Ex. 19                                                                              Ex. 20                                                                              Ex.                  __________________________________________________________________________                                                             21                   Conditions                                                                    Temperature (°C.)                                                                      226   227   252  251   248   248   249   248                  Pressure (psig) 1     1     1    1     1     1     1     1                    Catalyst        LZY-72                                                                              LZY-72                                                                              US-Y.sup.5                                                                         LZY-72                                                                              LZY-72                                                                              LZY-72                                                                              LZY-72                                                                              LZY-72               Feed/catalyst wt ratio                                                                        50.4  50.4  50.8 50.6  50.6  50.6  50.6  50.6                 Hours on stream 19.5  23.3  11.5 3.0   4.8   6.8   8.5   10.5                 Product Composition (wt %)                                                    1,2-DMN         0.0   0.0   0.0  0.0   0.0   0.0   0.0   0.0                  1,3-DMN         0.0   0.0   0.0  0.0   0.0   0.0   0.0   0.0                  1,4-DMN         0.0   0.0   0.0  0.0   0.0   0.0   0.0   0.0                  1,5-DMN         24.0  21.3  17.5 20.9  15.1  12.2  9.6   8.7                  1,6-DMN         39.9  40.3  41.5 41.6  41.6  41.6  40.0  39.7                 1,7-DMN         1.0   0.9   1.0  0.9   1.0   1.0   1.2   1.2                  2,3-DMN         0.0   0.0   0.0  0.0   0.0   0.0   0.0   0.0                  2,6- + 2,7-DMNs 31.8  34.2  35.9 32.9  37.6  40.0  43.0  43.3                 Lights          1.9   0.9   1.2  1.6   1.6   1.2   1.0   1.1                  Heavies         0.9   1.1   1.0  0.8   1.1   1.6   2.1   2.2                  Naphthalene     0.0   0.0   0.0  0.0   0.0   0.0   0.0   0.0                  Methylnaphthalenes                                                                            1.0   1.1   1.3  1.1   1.5   1.7   2.6   2.8                  Other           0.4   0.2   0.5  0.3   0.5   0.7   0.5   0.9                  Total           100.9 100.0 99.9 100.1 100.0 100.0 100.0 99.9                 Total DMNs      96.7  96.7  95.9 96.3  95.2  94.8  93.9  93.0                 2,7-DMN %       1.1         1.1                          1.6                  2,6-DMN % in the 1,5-, 1,6- and                                                               32.5  35.1  37.2 33.8  39.2  42.0  45.7  46.5                 2,6-DMN triad                                                                 2,6-DMN selectivity                                                                           99.5  99.9  97.3 98.4  95.6  94.6  92.3  90.5                 __________________________________________________________________________     Footnote                                                                      .sup.5 ultrastable sieve containing 2.6% Na                              

                                          Feed Ex. 22                                                                             Feed Ex.                                                                                Ex.                 __________________________________________________________________________                                                              24                                          Conditions                                                                    Temperature (°C.)                                                                         251       247  252                                         Pressure (psig)    1         1    1                                           Catalyst      LZY-72                                                                             LZY-72                                                                             LZY-72                                                                             LZY-72                                                                             LZY-72                                      Feed/catalyst wt ratio                                                                           50.0      44.0 44.0                                        Hours on stream    4.0       2.0  6.5                                         Product Composition (wt %)                                                    1,2-DMN       0.0  0.0  0.0  0.1  0.4                                         1,3-DMN       0.0  0.0  0.6  50.8 51.0                                        1,4-DMN       0.0  0.0  90.6 15.1 9.0                                         1,5-DMN       0.5  0.2  0.0  0.0  0.0                                         1,6-DMN       0.7  1.4  0.0  0.0  0.0                                         1,7-DMN       90.4 40.5 0.0  0.0  0.1                                         2,3-DMN       0.0  0.0  0.0  22.1 23.3                                        2,6-DMN       0.0  1.3  0.0  0.0  0.3                                         2,7-DMN       0.3  44.7 0.0  0.0  0.1                                         Lights        6.6  6.3  5.2  4.3  3.6                                         Heavies       0.0  0.7  0.0  1.8  3.8                                         Naphthalene             0.0  2.0  3.3                                         Methylnaphthalenes                                                                          0.2  1.7  3.6  3.9  4.8                                         Other         1.3  2.8  0.0  0.0  0.3                                         Total         100.0                                                                              99.6 100.0                                                                              100.1                                                                              100.0                                       Total DMNs    92.0 88.4 91.1 88.1 84.2                                        % desired DMN.sup.1 in its triad                                                            0.3  52.2 0.6  25.1 28.0                                        Selectivity        89.0      87.5 74.8                __________________________________________________________________________     .sup.1 2,7DMN in Example 22 and 2,3DMN in Examples 23-24.                

EXAMPLE 25

7.5 kilograms of distilled water, 7.5 kilograms of an aqueous solutioncontaining 40 weight percent of tetraethylamine hydroxide, 50 grams ofsodium hydroxide and 300 grams of sodium aluminate were stirred anddissolved in a 25-gallon stainless steel tank. The resulting solutionand 12.2 kilograms of a silica sol containing 40 weight percent ofsilica were mixed and stirred in a 10-gallon autoclave at 150° C. for 72hours. The resulting mixture was filtered, and the separated solids werewashed three times with distilled water, dried at 120° C. and thencalcined at 538° C. for 4 hours.

The resulting dried powder contained 0.37 weight percent of sodium,calculated as elemental sodium, and x-ray diffraction analysis indicatedthat the powder had the x-ray diffraction pattern of beta zeolite. Thefollowing is the x-ray diffraction pattern of the powder product,showing only the lines that are common to all 4 sources of beta zeolitein U.S. Pat. No. 3,308,069.

    ______________________________________                                                Line Relative                                                                 d(A) Intensity                                                        ______________________________________                                                4.18 16.2                                                                     3.99 100.0                                                                    3.54 6.1                                                                      3.35 12.6                                                                     3.11 3.0                                                                      3.05 14.6                                                                     2.94 5.3                                                                      2.69 4.1                                                                      2.54 1.5                                                                      2.08 11.5                                                             ______________________________________                                    

The powder was employed as the catalyst without being ion-exchanged Somepowder was ion-exchanged using the ion-exchange procedure of Example 27to reduce the sodium content, and after being ion-exchanged, thepowder's alumina content, silica-to-alumina mole ratio andsilicon-to-aluminum atom ratio were measured as 1.14 weight percent,68:1 and 34:1, respectively.

EXAMPLE 26

8 kilograms of distilled water, 8 kilograms of an aqueous solutioncontaining 40 weight percent of tetraethylamine hydroxide, 3.81kilograms of an aqueous solution containing 20 weight percent oftetraethylamine hydroxide, 0.6 kilogram of sodium aluminate, and 12.2kilograms of a silica sol containing 40 weight percent of silica weremixed and stirred in a 10-gallon autoclave at 150° C. for 72 hours. Theresulting mixture was filtered, and the separated solids were washedthree times with distilled water, dried at 120° C. for about 16 hoursand then calcined at 538° C. for 6 hours

The resulting dried powder contained 0.17 weight percent of sodium,calculated as elemental sodium. X-ray diffraction analysis indicatedthat the powder had the x-ray diffraction pattern of beta zeolite. Thefollowing is the x-ray diffraction pattern of the powder product,showing only the lines that are common to all 4 sources of beta zeolitein U.S. Pat. No. 3,308,069.

    ______________________________________                                                Line Relative                                                                 d(A) Intensity                                                        ______________________________________                                                4.19 17.7                                                                     4.01 100.0                                                                    3.54 Weak                                                                     3.35 13.8                                                                     3.11 Weak                                                                     3.05 13.4                                                                     2.95  2.8                                                                     2.67 Weak                                                                     2.49  0.6                                                                     2.09  7.6                                                             ______________________________________                                    

The powder was employed as the catalyst without being ion-exchanged.After being ion-exchanged using the procedure of Example 27 in order toreduce the sodium content, the powder's silica-to-alumina mole ratio andsilicon-to-aluminum atom ratio were measured as 30:1 and 14.8:1,respectively.

EXAMPLE 27

2.3 kilograms of the un-ion-exchanged catalyst powder produced inExample 26, 4 kilograms of distilled water, and 12 kilograms of anaqueous solution containing 19 weight percent of ammonium nitrate werestirred in a 22-liter flask at 72° C. for 4 hours. The mixture was thencooled; the liquid was removed by decantation, and the resultingion-exchanged catalyst was then washed with water. The catalyst was thendried at 120° C. and calcined at 538° C. for 3 hours. The ion-exchangedcatalyst contained 0.01 weight percent of sodium (calculated aselemental sodium), 2.43 weight percent of aluminum (calculated aselemental aluminum), and a silica-to-alumina mole ratio and asilicon-to-aluminum atomic ratio of 30:1 and 14.8:1, respectively.

163 grams of this dry, ion-exchanged beta zeolite powder, 454 grams ofan alumina sol containing 8.8 weight percent of solids, and 123 grams ofdistilled water were blended to obtain a smooth, uniform slurry. Theslurry was maintained at 23° C. for 5 hours to permit liquid toevaporate from the slurry. The slurry was then dried at 120° C. forabout 16 hours and calcined at 538° C. for 2 hours, to afford solidscontaining 80 weight percent of beta zeolite and 20 weight percent ofalumina, which were then ground and sieved to form particles having a20-40 mesh size.

EXAMPLES 28-46

In each of Examples 28-46, the particular feedstock employed was mixedin liquid form with a catalyst in a stirred reaction vessel with acontinuous nitrogen purge to preclude oxygen from the system. The weightratio of the feedstock-to-zeolite component of the catalyst was 49:1 ineach case. The pressure of the contents of the reaction vessel wasmaintained at about 1 pound per square inch gauge. The temperature ofthe reaction vessel was raised to the reaction temperature and sampleswere withdrawn at various times after commencement of the reaction andanalyzed. The conditions employed, the compositions of the feedstockemployed and of the resulting products, the percent of the 1,5-, 1,6-and 2,6-DMN triad in each thereof, the percent of 2,6-DMN in each such1,5-, 1,6- and 2,6-DMN triad, the percent decreases in each 1,5-, 1,6-and 2,6-DMN triad, the percent gain in each 1,7-, 1-8- and 2,7-DMN triadand the percent gain in total methylnaphthalene and trimethylnaphthalenecontent in each of Examples 28-46 are presented in Tables 3-7.

The catalyst employed in Examples 28-30 was commercially available UnionCarbide's unsupported LZ-Y72 in the hydrogen form as received from themanufacturer. The catalyst employed in Examples 31-34 was an unsupportedbeta zeolite having a relatively high silicon-to-aluminum ratio andprepared by the procedure of Example 25. The catalyst employed inExamples 35-43 was an unsupported beta zeolite having a relatively lowsilicon-to-aluminum ratio and prepared by the procedure of Example 26. Asingle sample of this catalyst was used for four cycles in Examples38-43. The catalyst employed in Examples 44-46 was also the beta zeolitehaving the relatively low silicon-to-aluminum ratio and prepared by theprocedure of Example 26, but in this instance was ion-exchanged toreduce the sodium content and supported on an alumina matrix by theprocedure of Example 27.

                  TABLE 3                                                         ______________________________________                                                    Feed  Ex. 28   Ex. 29   Ex. 30                                    ______________________________________                                        Conditions                                                                    Catalyst              LZ-Y72   LZ-Y72 LZ-Y72                                  Temperature (°C.)                                                                            250      250    250                                     Hours on Stream       1        3      4.75                                    Product Composition                                                           (wt %)                                                                        1,5-DMN       91.03   38.70    18.30  12.84                                   1,6-DMN       3.73    36.92    40.67  40.35                                   2,6-DMN       0       18.42    32.40  36.18                                   1,7-DMN       0.74    0.81     0.93   1.08                                    2,7-DMN       0       0.73     1.37   1.81                                    Methylnaphthalenes                                                                          0.06    0.62     1.43   2.03                                    Trimethylnaphthalenes                                                                       0.44    0.53     1.33   2.02                                    Other         4.00    3.27     3.57   3.69                                    Total         100.00  100.00   100.00 100.00                                  1,5-, 1,6- and                                                                              94.76   94.04    91.37  89.37                                   2,6-DMN triad content                                                         2,6-DMN percent in                                                                          0       19.59    35.46  40.49                                   1,5-, 1,6- and 2,6-DMN                                                        triad                                                                         1,5-, 1,6- and 2,6-DMN                                                                      0       0.72     3.39   5.39                                    triad percent loss                                                            1,7-, 1,8- and 2,7-DMN                                                                      0       0.80     1.56   2.15                                    triad percent gain                                                            Methylnaphthalene and                                                                       0       0.71     2.32   3.61                                    trimethylnaphthalene                                                          percent gain                                                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                   Feed  Ex. 31  Ex. 32  Ex. 33                                                                              Ex. 34                                 ______________________________________                                        Conditions                                                                    Catalyst from        25      25    25    25                                   Example                                                                       Temperature (°C.)                                                                           250     250   250   250                                  Hours on Stream      1       3     5     7                                    Product Composition                                                           (wt %)                                                                        1,5-DMN      91.03   54.00   28.93 18.94 14.09                                1,6-DMN      3.73    28.76   37.62 39.70 40.41                                2,6-DMN      0       12.91   28.76 35.96 39.55                                1,7-DMN      0.74    0.61    0.58  0.60  0.67                                 2,7-DMN      0       0.67    1.09  1.13  1.28                                 Methylnaphthalenes                                                                         0       0.12    0.29  0.41  0.57                                 Trimethylnaphthalenes                                                                      0.44    0.10    0.19  0.34  0.46                                 Other        4.06    2.83    2.67  2.92  2.97                                 Total        100.00  100.00  100.00                                                                              100.00                                                                              100.00                               1,5-, 1,6- and 2,6-                                                                        94.76   95.67   95.18 94.60 94.05                                DMN triad content                                                             2,6-DMN percent in                                                                         0       13.49   30.08 38.01 42.05                                1,5-, 1,6- and 2,6-                                                           DMN triad                                                                     1,5-, 1,6- and 2,6-                                                                        0       -0.91   -0.42 0.16  0.71                                 DMN triad                                                                     percent loss                                                                  1,7-, 1,8- and 2,7-                                                                        0       0.54    0.93  0.99  1.21                                 DMN triad                                                                     percent gain                                                                  Methylnaphthalene                                                                          0       -0.23   0.04  0.31  0.59                                 and trimethylnaphtha-                                                         lene percent gain                                                             ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                       Feed  Ex. 35  Ex. 36  Ex. 37                                   ______________________________________                                        Conditions                                                                    Catalyst from Example    26      26    26                                     Temperature (°C.) 250     250   250                                    Hours on Stream          1.25    3     5                                      Product Composition (wt %)                                                    1,5-DMN          91.03   21.94   10.79 8.20                                   1,6-DMN          3.73    38.62   40.89 41.20                                  2,6-DMN          0       35.59   43.28 44.94                                  1,7-DMN          0.74    0.52    0.60  0.64                                   2,7-DMN          0       0.30    0.53  0.47                                   Methylnaphthalenes                                                                             0       0.33    0.59  0.84                                   Trimethylnaphthalenes                                                                          0.44    0.16    0.46  0.78                                   Other            4.06    2.54    2.86  2.93                                   Total            100.00  100.00  100.00                                                                              100.00                                 1,5-, 1,6- and   94.76   96.15   94.96 94.34                                  2,6-DMN triad content                                                         2,6-DMN percent in 1,5-, 1,6-                                                                  0       37.02   45.58 47.63                                  and 2,6-DMN triad                                                             1,5-, 1,6- and 2,6-DMN                                                                         0       -1.39   -0.20 0.42                                   triad percent loss                                                            1,7-, 1,8- and 2,7-DMN                                                                         0       0.08    0.39  0.37                                   triad percent gain                                                            Methylnaphthalene and                                                                          0       0.05    0.61  1.18                                   trimethylnaphthalene                                                          percent gain                                                                  ______________________________________                                    

                                      TABLE 6                                     __________________________________________________________________________                  Feed                                                                              Ex. 38                                                                            Ex. 39                                                                            Ex. 40                                                                            Feed                                                                              Ex. 41                                                                            Ex. 42                                                                            Ex. 43                              __________________________________________________________________________    Conditions                                                                    Catalyst from Example                                                                           26  26  26      26  26  26                                  Temperature (°C.)                                                                        240 240 240     240 265 265                                 Hours on Stream   3   3.9 3       4.5 3   4.5                                 Catalyst Cycle    1st 1st 3rd     3rd 4th 4th                                 Product Composition (wt %)                                                    1,5-DMN       88.14                                                                             9.63                                                                              7.99                                                                              26.62                                                                             88.14                                                                             17.73                                                                             11.47                                                                             8.16                                1,6-DMN       3.66                                                                              39.45                                                                             39.53                                                                             35.39                                                                             3.66                                                                              38.10                                                                             39.23                                                                             39.73                               2,6-DMN       0   41.50                                                                             42.34                                                                             29.83                                                                             0   36.25                                                                             40.02                                                                             42.31                               1,7-DMN       0.74                                                                              0.66                                                                              0.69                                                                              0.57                                                                              0.74                                                                              0.59                                                                              0.66                                                                              0.72                                2,7-DMN       0   1.26                                                                              1.50                                                                              1.02                                                                              0   0.97                                                                              1.20                                                                              1.13                                Methylnaphthalenes                                                                          0.13                                                                              0.99                                                                              1.17                                                                              0.26                                                                              0.13                                                                              0.33                                                                              0.48                                                                              0.70                                Trimethylnaphthalenes                                                                       0.54                                                                              0.53                                                                              0.70                                                                              0.17                                                                              0.54                                                                              0.29                                                                              0.37                                                                              0.52                                Other         6.79                                                                              5.98                                                                              6.08                                                                              6.14                                                                              6.79                                                                              5.74                                                                              6.57                                                                              6.73                                Total         100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                              1,5-, 1,6- and 2,6-DMN                                                                      91.80                                                                             90.58                                                                             89.86                                                                             91.84                                                                             91.80                                                                             92.08                                                                             90.72                                                                             90.20                               triad content                                                                 2,6-DMN percent in 1,5-, 1,6-                                                               0   45.82                                                                             47.12                                                                             32.48                                                                             0   39.37                                                                             44.11                                                                             46.91                               and 2,6-DMN triad                                                             1,5-, 1,6- and 2,6-DMN                                                                      0   1.22                                                                              1.94                                                                              -0.04                                                                             0   -0.28                                                                             1.08                                                                              1.60                                triad percent loss                                                            1,7-, 1,8- and 2,7-DMN                                                                      0   1.18                                                                              1.45                                                                              0.85                                                                              0   0.82                                                                              1.12                                                                              1.11                                triad percent gain                                                            Methylnaphthalene and                                                                       0   0.85                                                                              1.20                                                                              -0.24                                                                             0   -0.05                                                                             0.18                                                                              0.55                                trimethylnaphthalene                                                          percent gain                                                                  __________________________________________________________________________

                  TABLE 7                                                         ______________________________________                                                       Feed  Ex. 44  Ex. 45  Ex. 46                                   ______________________________________                                        Conditions                                                                    Catalyst from Example    27      27    27                                     Temperature (°C.) 250     250   250                                    Hours on Stream          1       2     3                                      Product Composition (wt %)                                                    1,5-DMN          88.14   16.50   11.20 9.23                                   1,6-DMN          3.66    38.10   39.30 39.70                                  2,6-DMN          0       37.42   41.07 42.15                                  1,7-DMN          0.74    0.53    0.55  0.58                                   2,7-DMN          0       0.97    0.84  1.00                                   Methylnaphthalenes                                                                             0.13    0.52    0.69  0.83                                   Trimethylnaphthalenes                                                                          0.54    0.32    0.62  0.75                                   Other            6.79    5.64    5.73  5.76                                   Total            100.00  100.00  100.00                                                                              100.00                                 1,5-, 1,6- and   91.80   92.02   91.57 91.08                                  2,6-DMN triad content                                                         2,6-DMN percent in 1,5-, 1,6-                                                                  0       40.67   44.85 46.28                                  and 2,6-DMN triad                                                             1,5-, 1,6- and 2,6-DMN                                                                         0       -0.22   0.23  0.72                                   triad percent loss                                                            1,7-, 1,8- and 2,7-DMN                                                                         0       0.76    0.65  0.84                                   triad percent gain                                                            Methylnaphthalene and                                                                          0       0.17    0.64  0.91                                   trimethylnaphthalene                                                          percent gain                                                                  ______________________________________                                    

Comparison of the results in Tables 3-7 illustrates clearly that (1) theuse of a beta zeolite catalyst affords reduced losses of the 1,5-, 1,6-and 2,6-dimethylnaphthalene triad, reduced formation ofmethylnaphthalenes, trimethylnaphthalenes and the 1,7-, 1,8- and2,7-dimethylnaphthalene triad relative to the use of the LZ-Y72 zeolitecatalyst; and (2) the use of a beta zeolite catalyst either unsupportedor supported on a base material and having a relatively lowersilicon-to-aluminum ratio affords greater formation of2,6-dimethylnaphthalene and reduced losses of the 1,5-, 1,6- and2,6-dimethylnaphthalene triad relative to the use of a beta zeolitecatalyst having a relatively higher silicon-to-aluminum ratio andpermits the use of lower reaction temperatures or the use at a highertemperature of even a partially deactivated catalyst relative to the useof a LZ-Y72 zeolite catalyst.

EXAMPLES 47-54

In each of Examples 47-54 a fixed bed reactor was used to evaluate acontinuous process for the dehydrogenation of a feedstock containing1,5-dimethyltetralin (1,5-DMT) to yield mainly 1,5-dimethylnaphthalene(1,5-DMN). The fixed bed reactor used was a 13 centimeter long stainlesssteel tube having a 0.20 centimeter inside diameter and packed with 3.15grams of a catalyst containing 0.35 wt % platinum and 0.35 wt. % rheniumon a 16-25 mesh gamma alumina support. Preheated feedstocks having thecompositions shown in Tables 8 and 9 were pumped upflow through thecatalyst bed in the reactor tube at the reaction conditions listed inTables 8 and 9, respectively. Thermocouples installed in the reactorinlet and exit monitored the reaction temperature. Heating tape wrappedaround the reactor tube was used to maintain the set reactiontemperature. Reaction pressure was maintained to keep the feedstock andhydrocarbon products as liquids. Reactor effluent was collected in a 300ml pressure vessel maintained at 70°-100° C. to keep the product mixtureliquid. The hydrogen gas generated by the dehydrogenation reaction wasvented continuously from the collection vessel through a pressureregulator.

The selectivity and conversion percentages presented in Tables 8 and 9were calculated according to the following equations: (1,6-DMN is in the1,5-, 1,6-, 2,6-DMN triad and can be isomerized to 2,6-DMN;consequently, 1,6-DMT and 1,6-DMN wt. percentages are included in thefollowing equations.) ##EQU1##

Table 8 reports data for the single-pass conversion of the 1,5-DMTcontaining feedstock to 1,5-DMN containing product. Table 9 reports datafor the single-pass conversion of a 1,5-DMT containing feedstock thatwas distilled to remove heavy components before subjecting the feedstockto the dehydrogenation reaction. Table 9 also reports data for atwo-pass conversion of the 1,5-DMT containing feedstock. In the two-passconversion procedure the product stream from the single-passdehydrogenation reaction was collected and passed through the fixed beddehydrogenation reactor a second time. This two-pass procedure simulatesa process utilizing two or more fixed bed dehydrogenation reactors inseries wherein the hydrogen formed by the dehydrogenation reaction ispartially or totally vented between reactors.

                                      TABLE 8                                     __________________________________________________________________________                Feed                                                                              Ex. 47                                                                            Ex. 48                                                                            Ex. 49                                                                            Ex. 50                                                                            Ex. 51                                                                            Ex. 52                                    __________________________________________________________________________    Conditions                                                                    Hours on stream 436 441 482 503 957 1223                                      Catalyst cycle no.                                                                            2   2   2   2   2   2                                         Temperature (°C.)                                                                      401 400 400 401 400 400                                       Pressure (psig) 201 201 202 201 170 178                                       WHSV (grams feed/hr/                                                                          4.14                                                                              2.07                                                                              0.54                                                                              1.07                                                                              2.12                                                                              2.13                                      gram catalyst)                                                                Composition (wt %)                                                            1,5-DMT     84.66                                                                             4.56                                                                              3.90                                                                              1.90                                                                              3.51                                                                              3.17                                                                              7.59                                      1,6-DMT     0.91                                                                              0.07                                                                              0.12                                                                              0.21                                                                              0.15                                                                              0.06                                                                              0.09                                      2,6- + 2,7- + 1,7-DMT                                                                     0.27                                                                              0.00                                                                              0.00                                                                              0.06                                                                              0.00                                                                              0.00                                                                              0.00                                      2,8-DMT     0.68                                                                              0.31                                                                              0.16                                                                              0.00                                                                              0.16                                                                              0.20                                                                              0.00                                      2,5- + 1,8-DMT                                                                            0.31                                                                              0.00                                                                              0.11                                                                              0.22                                                                              0.00                                                                              0.08                                                                              0.23                                      Lights      0.24                                                                              1.68                                                                              2.36                                                                              9.61                                                                              3.31                                                                              2.05                                                                              1.54                                      C.sub.12 Indane                                                                           1.06                                                                              1.70                                                                              2.10                                                                              2.08                                                                              2.56                                                                              1.82                                                                              1.78                                      o-Tolylpentane                                                                            3.20                                                                              3.68                                                                              3.78                                                                              2.48                                                                              3.74                                                                              3.78                                                                              3.77                                      Products                                                                      1,5-DMN     1.64                                                                              80.15                                                                             77.48                                                                             51.95                                                                             71.46                                                                             78.97                                                                             75.68                                     1,6-DMN     0.07                                                                              2.37                                                                              4.18                                                                              11.20                                                                             6.54                                                                              3.72                                                                              3.09                                      1,7-DMN     0.00                                                                              0.42                                                                              0.62                                                                              1.29                                                                              0.78                                                                              0.59                                                                              0.50                                      1,8-DMN     0.00                                                                              0.18                                                                              0.11                                                                              0.04                                                                              0.04                                                                              0.05                                                                              0.18                                      2,6- + 2,7-DMN                                                                            0.00                                                                              0.00                                                                              0.10                                                                              0.79                                                                              0.23                                                                              0.09                                                                              0.16                                      1-Methylnaphthalene                                                                       0.00                                                                              0.57                                                                              1.15                                                                              7.60                                                                              2.09                                                                              0.79                                                                              0.65                                      2-Methylnaphthalene                                                                       0.00                                                                              0.00                                                                              0.00                                                                              0.29                                                                              0.10                                                                              0.06                                                                              0.05                                      Heavies     6.69                                                                              3.38                                                                              2.23                                                                              2.08                                                                              2.48                                                                              2.23                                                                              2.18                                      Others      0.27                                                                              0.94                                                                              1.62                                                                              8.22                                                                              2.85                                                                              2.33                                                                              2.52                                      Total       100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                            100.00                                    % Conversion                                                                              --  94.6                                                                              95.3                                                                              97.5                                                                              95.7                                                                              96.2                                                                              91.0                                      % Selectivity                                                                             --  99.8                                                                              98.0                                                                              73.6                                                                              93.1                                                                              98.3                                                                              98.9                                      __________________________________________________________________________

                  TABLE 9                                                         ______________________________________                                                         Ex. 53   Ex. 54                                                         Feed  (1st pass)                                                                             (2nd pass)                                                                              Overall                                   ______________________________________                                        Condition                                                                     Hours on stream      477      740                                             Catalyst cycle no.   3        3                                               Temperature (°C.)                                                                           400      401                                             Pressure (psig)      200      200                                             WHSV (grams feed/    4.30     4.36                                            hr/gram catalyst)                                                             Composition (wt %)                                                            1,5-DMT      83.16   7.78     1.05                                            1,6-DMT      2.79    0.20     0.04                                            2,6- + 2,7- +                                                                              0.87    0.06     0.00                                            1,7-DMT                                                                       2,8-DMT      0.82    0.45     0.06                                            2,5- + 1,8-DMT                                                                             0.87    0.20     0.05                                            Lights       0.46    1.34     1.49                                            C.sub.12 Indane                                                                            1.52    1.71     1.65                                            o-Tolylpentane                                                                             4.88    5.04     4.67                                            Products                                                                      1,5-DMN      2.02    76.00    79.30                                           1,6-DMN      0.36    4.51     7.17                                            1,7-DMN      0.03    0.88     1.24                                            1,8-DMN      0.01    0.17     0.12                                            2,6- + 2,7-DMN                                                                             0.08    0.18     0.33                                            1-Methylnaphthalene                                                                        0.00    0.28     1.19                                            2-Methylnaphthalene                                                                        0.06    0.01     0.05                                            Heavies      0.17    0.45     0.80                                            Others       1.70    0.75     0.78                                            Total        100.00  100.00   100.00                                          % Conversion --      90.7     86.4    98.7                                    % Selectivity                                                                              --      100.0    86.5    98.9                                    ______________________________________                                    

Examples 47-54 demonstrate that high conversions are possible at a widerange of flow rates (WHSV) of from about 0.54 to about 4.14 grams offeed per gram of catalyst per hour using a continuous dehydrogenationprocedure. As the flow rates increase the conversion decreases; however,the selectivity to 1,5- and 1,6-DMN increases. Example 51 and 52demonstrate that even after many hours of liquid phase operation highconversion and selectivities are maintained at a reaction temperature ofabout 400° C. Noble metal catalysts in general require the addition oflarge amounts of hydrogen to maintain catalytic activity for prolongedperiods. However, by employing reaction conditions in thedehydrogenation method of this invention such that a liquid reactionmixture was maintained, additional hydrogen was not required to maintaincatalyst activity. In commercial operation, expensive hydrogen recycleequipment would not, therefore, be required. Examples 53 and 54demonstrate that the continuous dehydrogenation reaction can be run inat least a two-pass process wherein the product stream from the firstpass through the reactor, after the removal of hydrogen, is feed to asecond fixed bed reactor. This type of operation can promote thedehydrogenation reaction by providing for the removal of hydrogenbetween reactors and thereby increasing the conversion and selectivityof the dehydrogenation reaction of this invention. By using thistwo-pass process conversion was increased from 90.7% for the first passto 98.7% after the second pass, as demonstrated by a comparison of thedata from Example 53 in Table 9 to the overall conversion reported inTable 9.

From the above description, it is apparent that the objects of thepresent invention have been achieved. While only certain embodimentshave been set forth, alternative embodiments and various modificationswill be apparent from the above description to those skilled in the art.These alternatives are considered equivalents and are within the spiritand scope of the present invention.

Having described the invention, what is claimed is:
 1. A method forpreparing a dimethylnaphthalene, comprising: contacting a firstfeedstock comprising at least one dimethyltetralin in liquid form with asolid dehydrogenation catalyst in a reaction vessel at an elevatedtemperature and at a pressure that is sufficiently high to maintain thefirst feedstock substantially in the liquid phase, to thereby effectconversion of the aforesaid dimethyltetralin in an equilibriumdehydrogenation reaction to form hydrogen and a first liquid productcomprising a dimethylnaphthalene formed from each aforesaiddimethyltetralin, and removing a substantial portion of the hydrogenbeing formed in the dehydrogenation reaction from the reaction vessel tothereby shift the aforesaid equilibrium toward the formation of thedimethylnaphthalene product.
 2. The method of claim 1 wherein either (a)1,5-, 1,6-, 2,5-, or 2,6-dimethyltetralin or a mixture thereof comprisesat least 80 weight percent of the dimethyltetralin content of the firstfeedstock, and 1,5-, 1,6- or 2,6-dimethylnaphthalene or a mixturethereof comprises at least 80 weight percent of the methylnaphthalenecontent of the first liquid product, (b) 1,5-, 1,6-, 1,7-, 1,8-, 2,5-,2,6-, 2,7-, or 2,8-dimethyltetralin or a mixture thereof comprises atleas. 80 weight percent of the dimethyltetralin content of the firstfeedstock and 1,5-, 1,6-, 1,7-, 1,8-, 2,6-, or 2,7-dimethylnaphthaleneor a mixture thereof comprises at least 80 weight percent of thedimethylnaphthalene content of the first liquid product, (c) 1,7-, 1,8-,2,7- or 2,8-dimethyltetralin or a mixture thereof comprises at least 80weight percent of the dimethyltetralin content of the first feedstockand 1,7-, 1,8- or 2,7-dimethylnaphthalene or a mixture thereof comprisesat least 80 weight percent of the dimethylnaphthalene content of thefirst liquid product, or (d) 1,3-, 1,4-, 2,3-, 5,7-, 5,8- or6,7-dimethyltetralin or a mixture thereof comprises at least 80 weightpercent of the dimethyltetralin content of the first feedstock and 1,3-,1,4- or 2,3-dimethylnaphthalene or a mixture thereof comprises at least80 weight percent of the dimethylnaphthalene content of the first liquidproduct.
 3. The method of claim 2 wherein 1,5-, 1,6-, 2,5-, or2,6-dimethyltetralin or a mixture thereof comprises at least 80 weightpercent of the dimethyltetralin content of the first feedstock and 1,5-,1,6- or 2,6-dimethylnaphthalene or a mixture thereof comprises at least80 weight percent of the dimethylnaphthalene content in the first liquidproduct.
 4. The method of claim 2 wherein 1,5-, 1,6-, 1,7-, 1,8-, 2,5-,2,6-, 2,7-, or 2,8-dimethyltetralin or a mixture thereof comprises atleast 80 weight percent of the dimethyltetralin content of the firstfeedstock and 1,5-, 1,6-, 1,7-, 1,8-, 2,6-, or 2,7-dimethylnaphthaleneor a mixture thereof comprises at least 80 weight percent of thedimethylnaphthalene content of the first liquid product.
 5. The methodof claim 2 wherein 1,7-, 1,8-, 2,7or 2,8-dimethyltetralin or a mixturethereof comprises at least 80 weight percent of the dimethyltetralincontent of the first feedstock, and 1,7-, 1,8- or2,7-dimethylnaphthalene or a mixture thereof comprises at least 80weight percent of the dimethylnaphthalene content of the first liquidproduct.
 6. The method of claim 2 wherein 1,3-, 1,4-, 2,3-, 5,7-, 5,8-or 6,7-dimethyltetralin or a mixture thereof comprises at least 80weight percent of the dimethyltetralin content of the first feedstock,and 1,3-, 1,4- or 2,3-dimethylnaphthalene or a mixture thereof comprisesat least 80 weight percent of the dimethynaphthalene content of thefirst liquid product.
 7. The method of claim 2 wherein thedehydrogenation is performed at a temperature in the range of from about200° C. to about 500° C.
 8. The method of claim 2 wherein thedehydrogenation is performed at a pressure in the range of from about0.1 to about 30 atmospheres absolute.
 9. The method of claim 2 whereinthe dehydrogenation catalyst comprises a noble metal component supportedon a substantially inert support material, with the noble metalcomponent employed at a level of from about 0.5 to about 15 weightpercent, calculated as the elemental noble metal and based on the weightof the dehydrogenation catalyst.
 10. The method of claim 9 wherein thenoble metal component comprises palladium.
 11. The method of claim 9wherein the dehydrogenation is performed continuously with a spacevelocity in the range of from about 0.1 to about 5000 parts of thefeedstock per part of the noble metal component (calculated as theelemental noble metal) of the dehydrogenation catalyst by weight perhour.
 12. The method of claim 9 wherein the dehydrogenation is performedon a batch basis with the dehydrogenation catalyst at a level in therange of from about 0.005 to about 1.0 percent of the noble metalcomponent, calculated as the elemental noble metal, and based on theweight of the dimethyltetralin feedstock and the reaction time is fromabout 1 to about 50 hours.
 13. The method of claim 2 wherein the secondfeedstock is dissolved in a solvent.
 14. The method of claim 13 whereinthe solvent is a paraffin or aromatic hydrocarbon which boils aboveabout 270° C.
 15. The method of claim 2 comprising the additional stepsof: contacting the aforesaid first liquid product in liquid form with asolid isomerization catalyst comprising beta zeolite or an acidicultrastable Y-crystalline zeolite having a silica-to-alumina molar ratioof from about 4 to about 10, and having pore windows provided bytwelve-membered rings containing oxygen and a unit cell size of fromabout 24.2 to about 24.7 angstroms, and at an elevated temperature andat a pressure that is sufficiently high to maintain the isomerizationfeedstock substantially in the liquid phase wherein (1) when at least 80weight percent of the dimethylnaphthalene content of the first liquidproduct comprises at least one of 1,5-, 1,6-, and2,6-dimethylnaphthalene, at least 20 weight percent of the total of 1,5-and 1,6-dimethylnaphthalenes is isomerized to 2,6-dimethylnaphthalene,(2) when at least 80 weight percent of the dimethylnaphthalene contentof the first liquid product comprises at least one of 1,5-, 1,6-, 1,7-,1,8-, 2,6- and 2,7-dimethylnaphthalenes, at least 20 weight percent ofthe total of 1,5-, 1,6-, 1,7- and 1,8-dimethylnaphthalenes is isomerizedto 2,6- and 2,7-dimethylnaphthalenes, (3) when at least 80 weightpercent of the dimethylnaphthalene content of the first liquid productcomprises at least one of 1,7-, 1,8- and 2,7-dimethylnaphthalene, atleast 20 weight percent of the total of 1,7- and1,8-dimethylnaphthalenes is isomerized to 2,7-dimethylnaphthalene, and(4) when at least 80 weight percent of the dimethylnaphthalene contentof the first liquid product comprises at least one of 1,3-, 1,4- and2,3-dimethylnaphthalenes, at least 20 weight percent of the total of1,3- and 1,4-dimethylnaphthalenes is isomerized to2,3-dimethylnaphthalene.
 16. The method of claim 15 wherein at least 25weight percent of the total of 1,5- and 1,6-dimethylnaphthalenes in theaforesaid first liquid product in (1) of claim 15 is isomerized to2,6-dimethylnaphthalene.
 17. The method of claim 15 wherein at least 25weight percent of the total of 1,5-, 1,6-, 1,7- and1,8-dimethylnaphthalenes in the aforesaid first liquid product in (2) ofclaim 15 is isomerized to 2,7-dimethylnaphthalene and2,6-dimethylnaphthalene.
 18. The method of claim 15 wherein at least 25weight percent of the total of 1,7- and 1,8-dimethylnaphthalenes in theaforesaid first liquid product in (3) of claim 15 is isomerized to2,7-dimethylnaphthalene.
 19. The method of claim 15 wherein at least 25weight percent of the total of 1,3- and 1,4-dimethylnaphthalenes in theaforesaid first liquid product in (4) of claim 15 is isomerized to2,3-dimethylnaphthalene.
 20. The method of claim 15 wherein theisomerization is performed at a temperature in the range of from about200° C. to about 420° C.
 21. The method of claim 15 wherein theisomerization is performed on a batch basis.
 22. The method of claim 15wherein the isomerization catalyst employed comprises beta zeolite. 23.The method of claim 22 wherein the isomerization catalyst comprises ahydrogenation component comprising a Group VIII metal.
 24. The method ofclaim 23 wherein the Group VIII metal is palladium, platinum or nickel.25. The method of claim 21 wherein the isomerization catalyst employedis free of a support material.
 26. The method of claim 21 wherein theisomerization catalyst is supported on an inorganic support material.27. The method of claim 26 wherein the support material comprisessilica, alumina, silica-alumina, or bentonite, or magnesia, or a mixturethereof.
 28. The method of claim 15 wherein the isomerization isperformed at a pressure in the range of from about 0.1 to about 20atmospheres absolute.
 29. The method of claim 15 wherein theisomerization is performed on a continuous basis with a space velocityof, or on a batch basis with an effective space velocity of, from about0.1 to about 20 parts of feedstock per part of the zeolite component ofthe isomerization catalyst by weight per hour.
 30. The method of claim 1wherein the dehydrogenation catalyst comprises a mixture of platinum andrhenium supported on an alumina support.
 31. The method of claim 30wherein the platinum and rhenium are each present in an amount of about0.01 to about 10.0 weight percent calculated based on the weight of thecatalyst and wherein the alumina support comprises gamma alumina. 32.The method of claim 1 wherein the dehydrogenation reaction is performedcontinuously using at least two series arranged fixed bed reactors andwherein hydrogen is removed from the liquid product between the fixedbed reactors.
 33. The method of claim 1 wherein said reaction vessel isa fixed bed reactor.